For example, in order to form a logic element of a semiconductor device, there may be a case where a process of forming a silicon layer is performed by supplying a film forming gas containing silicon to a surface of a semiconductor wafer (hereinafter referred to as a wafer) as a substrate, and filling silicon into a recess formed downward in the surface of the wafer.
It is sometimes the case that the recess is formed so that the top portion of the inner side wall thereof bulges out and an opening width at an upper side in the recess is smaller than an opening width at a lower side. If the film forming gas is supplied to the recess having such a shape for a relatively long period of time, the upper portion of the recess is closed before the filling of silicon into the recess is completed, whereby voids may be included in a silicon layer formed inside the recess. Thus, silicon may sometimes be filled into the recess by a method called DED (Deposition Etch Deposition). In this method, a first-round film forming process of primarily supplying a film forming gas, an etching process of supplying an etching gas, and a second-round film forming process of secondarily supplying the film forming gas are performed in this order. For example, a process is used to plasmarize an etching gas, which is a mixed gas of a chlorine gas and a hydrogen bromide gas, and etch silicon existing on a surface of a wafer.
Describing the above DED method in more detail, the first-round film forming process is completed before the clogging of the upper portion of the recess occurs, thereby obtaining a state in which a silicon film is formed inside the recess. The subsequent etching process is performed so that the opening width at the upper side in the recess is enlarged and the silicon film remains in the recess. In the second-round film forming process after the etching is performed in this manner, silicon is filled into the recess while suppressing the formation of the aforementioned voids.
In order to perform the etching process described above, as the etching gas, a gas having relatively high reactivity with silicon, for example, a chlorine gas, is used so as to react with the silicon film at the upper side of the recess before reaching the lower side of the recess. However, if the etching is performed with this chlorine gas, the roughness of the surface of the silicon film becomes relatively large. Since the reason will be described in detail in the Detailed Description section of the present disclosure, brief descriptions are offered here. If silicon is deposited on the surface of the silicon film having relatively large roughness during the second-round film forming process, there is concern that micro voids are included in a silicon layer formed inside the recess, or seams are formed in the silicon layer. When subjecting the silicon layer to anisotropic etching, the etching may progress greatly because the density of silicon is lower at locations where the voids or seams are formed than at other locations.